A secondary battery repeats charging and discharging as chemical energy and electric energy are interconverted each other by chemical reactions of oxidation and reduction. The secondary battery generally includes four basic elements, that is, an anode, a cathode, a separator and an electrolyte. Herein, the anode and the cathode are also referred to as an electrode, and the electrode includes active materials causing an actual reaction.
For example a lithium ion secondary battery uses a liquid electrolyte. However, the liquid electrolyte may have disadvantages, since it is volatile, such as a danger of explosion, and inferior thermal stability.
Meanwhile, an all solid state battery using a solid electrolyte may have low danger of explosion, and also have improved thermal stability. In addition, when a bi-polar plate is used, a high operating voltage may be obtained by laminating electrode through a series connection, such that higher energy density may be obtained compared to the energy density of the battery that uses the liquid electrolyte in a series of cell connection.
The all solid state battery requires a solid electrolyte transferring lithium ions. The solid electrolyte may be classified largely into an organic (polymer) electrolyte and an inorganic electrolyte, and the inorganic electrolyte may include an oxide-based electrolyte and a sulfide-based electrolyte.
For instance, the oxide-based solid electrolyte may include oxygen, such as a LiPON-type, a perovskite-type, a garnet-type and a glass ceramic-type, and have an ionic conductivity of 10−5 to 10−3 S/cm that is less than that of a sulfide-based electrolyte. Meanwhile, the oxide-based solid electrolyte may provide advantages such that an oxide-based solid electrolyte can be stable with respect to moisture and have low reactivity with oxygen in the atmosphere, when compared to a sulfide-based solid electrolyte.
However, the oxide-based solid electrolyte may have high grain boundary resistance, and therefore, an electrolyte membrane or pellets, in which necking between the particles are formed from high temperature sintering, may be used. Further, the oxide-based solid electrolyte may not be produced in large amount, since a large-area electrolyte membrane thereof may not be formed due to high temperature of sintering, i.e. from about 900 to about 1400° C.
Particularly, the garnet-type electrolyte among the oxide-based solid electrolytes may strictly require a long time for about 6 hours or longer at a temperature of 1000 to 1250° C. in final calcination. In addition, in order to prevent lithium volatilization and to secure phase changes and composition uniformity, pellet-covered garnet may be used as imitation. However, the proportion of the garnet obtained using such pellets may be usually less than 20% by weight with respect to the total weight, which is considered as being inefficient.
The US Patent Application Laid-Open Publication No. 2013-0344416 has disclosed solid oxide ceramics prepared by hot pressing pellets that may be prepared including lithium carbonate, lanthanum hydroxide, zirconium oxide and alumina, however, the thus prepared pellet-type LLZ may be formed to have low crystallizability.
Accordingly, in order to prepare the garnet having excellent physical properties, many researches including basic physical property studies and garnet preparation have been progressed, and furthermore, development on the materials capable of being utilized in a manufacturing process of all solid state batteries has been required.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.